Tires, including tire components, must typically be cured under pressure beyond what is known as the blow point, which is the point of cure of the rubber compound where blows no longer form. Blows generally are small or large bubbles, or blisters, that are formed in the rubber compound as a result of the formation and expansion of gases trapped therein during curing. In a typical pressure molding process, e.g. compression or injection molding, the external pressure from the mold helps prevent the gases from expanding. At a certain point during vulcanization or crosslinking of the rubber compound, i.e. the blow point, the modulus of the rubber compound prevents further expansion of the gases, without pressure from the curing mold. In typical rubber compounds, the blow point is a level of curing of those compounds that is usually at least 30% of full cure thereof. At this point, the tire can be removed from the mold.
In a tire, the cure time typically is set so that the point of least cure exceeds the blow point. The location of least cure is usually found at the thickest part of the tire. In order to increase the production rate of pressure-cured tires, it would be desirable to lower the curing time. Lowering the blow point will reduce the cure time of tires cured under pressure, allowing for increased tire production. As indicated above, the blow point is the extent of cure of the rubber compound of interest wherein gas expansion generated during pressure curing of the rubber compound no longer occurs. Normally, long cure times under pressure, e.g. 30% to 100% of the full cure of the rubber compound, are required for the elimination of blows in curing tires or tire components. In pressureless cures, such as high temperature salt baths, high levels of a desiccant, e.g. at least about 12 phr, are required for the elimination of blows. PHR is used herein to describe parts by weight per 100 parts by weight of rubber in the rubber compound.
Accordingly, there remains a need for an improved method for reducing the curing time of tires cured under pressure to increase the production rate thereof.